The present invention relates to fine wire feeding mechanisms of the type used with automatic semiconductor wire bonders. More particularly, the present invention relates to a combined wire tensioning and slack reserve apparatus used in a wire feed mechanism of an automatic wire bonder.
Automatic wire bonders are known in the semiconductor manufacturing industry. A commercially available processor controlled automatic wire bonder is made by Kulicke and Soffa Industries, Inc. (the assignee of the present invention) and is shown and described in U.S. Pat. No. 4,266,710.
Modern automated wire bonders employ a wire-feed system whose two primary purposes are to (a) ensure that wire is smoothly and continuously delivered from its spool to the bonding tool where the wire is continually being consumed by the bonding process and (b) place an upward tension force on the wire during certain portions of the bonding cycle to aid in seating the wire into the bonding capillary. These two requirements have conventionally been satisfied by employing two independent sub-systems of the wire feed system, one to ensure smooth continuous feeding of the wire and the other to apply tension to the wire.
To ensure smooth and continuous wire feeding, a xe2x80x9cwire slack reservoirxe2x80x9d or xe2x80x9cair guidexe2x80x9d of the type shown and described, for example, in U.S. Pat. Nos. 4,736,826 and 5,564,616 are usually employed. The main characteristic and operating principles of such conventional slack reservoirs are the supply of a pressurized fluid transverse to the direction of the wire from a nozzle, which is supplied with compressed fluid. The wire may be constrained between two closely spaced plates, forming a channel through which the fluid flows, or a single fluid/wire guiding surface may be used.
The flow of pressurized fluid transverse to the wire axis exerts a bi-directional tension force on the wire which, in turn, causes the wire to form a bent section or xe2x80x9cloopxe2x80x9d, the size and shape of which depends upon the amount of fluid flow supplied to the device. As wire is consumed in the bonding process, the slack loop continually decreases in size at a rate dependant upon the amount of wire forming each bond as well as the number of bonds produced in unit time.
One or more sensors (optical or other type) are typically employed in slack reservoir systems to sense the decreasing wire loop, and to re-supply the reservoir and re-establish a full-sized loop with additional wire from the wire supply spool. There is typically an electronic communication and synchronization between the sensor(s) and a motor to which the wire spool is attached for coordination of wire spool rotation and wire slack loop re-supply. In this way, wire is continuously supplied to the bonder when required and no supply occurs when the bonder is idle.
One undesirable feature of conventional wire slack systems, which employ transverse fluid flows, is that in producing the wire loop, a bi-directional tension is also supplied to the wire. The actual amount of tension depends upon the amount of fluid flow supplied to the device as well as on the loop shape, or amount of slack reserve in the unit. This variable tension is undesirable because during the bonding process, the wire slack loop naturally shrinks and grows in size, and hence, a variable amount of tension is applied to the wire during bonding. This variable component of tension is in addition to the steady tension supplied by the separate tensioner unit. Varying tension leads to undesirable bonding results such as variable loop heights and potentially tight (abnormally low) loops. The sensitivity of conventional transverse flow systems to the loop size and shape can be severe.
The second component of conventional wire feed systems is a wire tensioner, the primary purpose of which is to supply a steady, pre-determined amount of tension to the wire during parts of the wire bond cycle. The amount of tension is typically adjustable by the amount of pressurized fluid which is supplied to the tensioner.
Most modern wire bonders utilize a tensioning system composed of a series of small hollow tubes, stacked end to end, and housed in an enclosing plenum structure. The wire is threaded through the inner surfaces of the multiple tubes.
In alternate types of systems, compressed fluid is either forced through the tubes (pressure type) or drawn in from the tubes (vacuum type) to establish a fluid flow through the tubes. The movement of this fluid flow along the wire produces a tension force on the wire through viscous and pressure type forces. The directionality of the tension force produced on the wire is ensured by making the inside diameters of the various tubes in the stack larger or smaller, thus presenting the fluid with a higher and lower resistance path. The predominant tension is produced on the wire by the fluid which flows along the low resistance path of the tensioner. Typical inner diameters for tensioner tubes can range from 0.020 inches for the low resistance path to 0.005 inches for the high resistance path.
Tensioners as described above have been used on automated wire bonders for many years and are commercially available. While these types of tensioners have proven convenient, they have a number of drawbacks which are becoming more and more apparent as the diameter of the wire used in wire bonding continues to shrink. Some of the known disadvantages of these conventional tensioning systems are:
a) Difficulty in threading the small diameter wire through the multiple tubes of the tensioner. When the wire encounters any sharp edge along the inner surfaces of the tubes, or interfaces between adjoining tubes, it has a tendency to bend. Because of the small inner diameters required for proper operation of the tensioner, the bent wire can no longer fit through the tubes and threading is prevented.
b) Fluid flow through the tensioner tubes, as well as the action of bonding, causes the wire to thrash back and forth and move up and down within the tensioner tubes. Because of this physical contact between the wire and the inner surfaces of the tensioner tubes, extremely smooth surface finishes are required on the inner surfaces of the tubes to prevent scratches from damaging the surface of the relatively soft bonding wire. It is very difficult however, to polish interior surfaces, especially cylindrical holes of dimensions typical of tensioner tubes and this relates to either inferior tubes which do indeed damage the wire, or prohibitively expensive tubes which posses the required surface interior surface finishes.
c) Inside surfaces of the tubes tend to become contaminated with time and/or usage due to the continual passage of wire through the tensioner tubes, as well as the flow of fluid. Cleaning such small tubes is difficult because of the restricted access to the inside surfaces. Typically, immersion in a cleaning solvent in an ultrasonic cleaner is required to properly clean the tubes. This is a time consuming process, which requires specialized equipment, and the need for disassembly and re-assembly can lead to occasional lost or damaged parts.
d) Conventional tube tensioning systems tend to produce an undesirable twisting moment on the wire in addition to the desired upwards tension. The origin of the twist force is not fully understood, but it is believed to originate from fluid swirling in a vortex motion within the tubes. The net result of the twisting force is a physical twisting or spinning of the wire during bonding. This leads to a condition known as xe2x80x9cleaning wirexe2x80x9d. In its severest form, leaning wires can touch one another and cause electrical shorting of the device being bonded.
Accordingly, it would be desirable to provide a uni-directional tensioning device that will virtually eliminate known problems associated with convention bonding wire tensioning systems.
In view of the shortcomings of the prior art, the present invention is directed towards removing the known limitations and significantly enhancing the performance and ease of use of existing wire-feed systems. The present invention is a device for simultaneously providing both a slack reserve loop of bonding wire and for providing a uni-directional upward tension on the wire without the known deficiencies of conventional wire feed system components.
The device includes an outlet port providing fluid under pressure, and an adjacent, continuously or stepped (piecewise linear) curved, surface to which the exhaust jet adheres through pressure and viscous drag forces. As the exhaust fluid is diverted along the surface, it develops into a highly accelerated, high velocity, low-pressure fluid jet which flows substantially along the guiding surface. The bonding wire is trapped by the low pressure within the exhaust jet and essentially follows the flow. The flow of high velocity fluid along the wire produces a uni-directional tension on it in the direction of the flow.
At the same time, because the wire is constrained by other surfaces of the device, the exhaust fluid is forced to flow transverse to a portion of the wire, creating a reserve loop. However, because most of the tension produced on the wire comes from the high velocity fluid flow which immediately adjacent to the exhaust nozzle, the tension force is much less sensitive to the amount of wire in the slack loop than conventional slack loop systems.
According to another aspect of the invention, a tensioning unit includes a fluid inlet port; a fluid reservoir coupled to the fluid inlet port; a fluid outlet port coupled to the fluid reservoir, the fluid flowing through the outlet port under pressure; a surface having a first end adjacent the outlet port, the pressurized fluid flowing along the surface from the first end and toward a second end of the surface, such that the pressurized fluid provides a uni-directional tension to a bonding wire following the surface of the tensioning unit.
According to yet another aspect of the invention, the surface includes a first curved portion at the first end; and a substantially linear portion coupled to the curved portion, such that the fluid exits the outlet port and follows the curved portion and a portion of the linear portion of the surface.
According to a further aspect of the invention, a method for provided a uni-directional tension to a bonding wire is disclosed. The method comprising the steps of providing a fluid to a reservoir; pressurizing the fluid; providing a curved surface adjacent the reservoir; and ejecting the pressurized fluid from the reservoir in a first direction along the curved surface, such that the bonding wire follows a path along the surface and the pressurized fluid provides a uni-directional tension to the bonding wire.
These and other aspects of the invention are set forth below with reference to the drawings and the description of exemplary embodiments of the invention.